Method of manufacturing chipboards, fibre boards and the like boards

ABSTRACT

A method of manufacturing plates, such as chipboards, fibre boards and the like boards, where the raw material in form of biomass particles, such as wood chips, wood fibres and the like applied with a thermosetting binder is spread onto a preforming band into an endless mat, and where this mat (B) is pre-compressed in a continuously operating prepress (C ) and then completely pressed in a continuously operating hot press, whereby the mat (B) is compressed into the desired thickness of the finished plate and the thermosetting binder is hardened. According to the invention, the mat (B) is pretreated by means of steam immediately before the introduction into the hot press (E) by means of a device (F) so as to obtain predetermined gradients of moisture content and temperature across the thickness of the mat. As a result the capacity of the apparatus can be increased at the same time as the energy consumption can be reduced. Furthermore, the dimensions and hydraulics of the press (E) can be reduced to a predetermined capacity. Finally, the possibility of controlling the total processing is improved concerning achievement of predetermined qualities of the finished plate characterized by the density profile of said plate across the thickness of said plate.

TECHNICAL FIELD

[0001] The invention relates to a method of optimizing the productioncapacity and the flexibility of the product properties whenmanufacturing chipboards, fibre boards and the like boards by acontinuous process, where a thermosetting binder is applied onto the rawmaterial in form of biomass particles, such as chips, fibres and thelike, said raw material being spread on a preforming band to form anendless mat, where said mat is preferably pre-compressed in acontinuously operating prepress and finally pressed in a continuouslyoperating hot press in such a manner that said mat is compressed intothe desired thickness of the finished plate and the thermosetting binderis hardened.

BACKGROUND ART

[0002] Above all, the hot press is essential to the production capacityof an apparatus and for the properties of the product, said hot presshaving two basic functions viz. to compress a mat of biomass particlesglued to the desired thickness of the plate and to heat said mat to atemperature causing a hardening, i.e. a polymerisation/condensation ofthe binder.

[0003] For this purpose, two types of hot presses are used, viz.conventional step presses pressing a section of the mat per pressingcycle and continuously operating through-type presses advancing anendless mat by means of steel bands through a wedge-shaped slot betweentwo pressing planes with the result that said mat is graduallycompressed and full-hardened by means of heat from said pressing planesand said steel bands. These modern presses become more and moreimportant and they are expected to dominate the market. The invention isin particular directed towards a use in connection with this type ofpress.

[0004] Below reference is only made to a continuous press, and thecapacity of said press depends on

[0005] the capacity of the press for transferring heat from the pressingplanes to the steel bands. In this connection, the shape of the rolleror slide systems between the pressing planes and the steel bands is ofdecisive importance, and

[0006] the transfer of heat from the steel band to the mat of woodparticles and through said mat which is to be heated to approximately105 to 110° C. in the middle in order to harden the binder.

[0007] In practice, the heat transfer in the mat turns out to be thelimiting factor. The thermal conductivity in the mat is very poor, andaccordingly attempts have been made at optimizing the so-called“Dampfstoss-Effekt”, which is a German technical term meaning that themoisture in the surface of the mat evaporates and moves towards thecentre of the mat where the steam condenses and releases its evaporationheat.

[0008]FIG. 4 shows an example of the temperature course at fourdifferent depths of the mat versus the time and consequently theposition of the measuring location above the pressing length. The curvesegments with a 20 steep temperature gradient represent the“Dampfstoss-Effekt” in the layer in question. The flat temperaturegradients represent the heat conducting phase taking over when steam isno longer supplied from the outside.

[0009] It appears that the heat conducting phase requires most time andrestricts the advancing speed and consequently the capacity of thepress.

[0010] Thus the “Dampfstoss-Effekt” is the ideal mechanism fortransferring heat.

[0011] It is, however, subject to limitations because a high steampressure in the middle layer may cause steam burstings in said middlelayer when the plate is leaving the press. The more water/steam that issupplied for heating the mat, the more time the plate must remain undera slight pressure in the press so that the steam can finally condense orescape from the middle layer.

[0012] Thus an optimizing of the capacity of the press by means ofwater/steam dosing presents a compromise between two counter-actingeffects.

[0013] The conventional method of pressing chipboards or fibre mats in acontinuous hot press has, however, not only a limiting effect on thecapacity of the press, but also a negative effect on the properties ofthe product.

[0014] The latter situation has been illustrated in the followingexample showing a conventional pressing of a fibre mat into an MDFplate, cf. FIG. 6A.

[0015] A precompressed 80 mm thick mat of glued wood fibres with amoisture content of 9 to 10% corresponding to a 16 mm thick MDF plate isintroduced in a continuous press and subjected to a compressing in thefirst section of the press by means of a very high pressure, usually ofthe magnitude of 40 to 50 kp/cm², into a thickness usually being 5 to10% smaller than the final thickness of the plate, cf. FIG. 6A-2. FIG.6A-2 shows the distance of the pressing planes, i.e. the thickness ofthe mat, over the length of the press, and FIG. 6A-1 shows the specificpressure in the mat over the length of the press.

[0016] The high pressure in the first phase and the heating from thepress bands (t˜200° C. or more) result in a plastifying and compressionof the fibres in the outermost layer of the mat into a density often inthe range of 1000 to 1100 kg/m³ for standard MDF-plates.

[0017] The pressure is then reduced in the second phase to for instance1 to 3 kp/cm² so as to improve the permeability of the middle layer tothe steam penetrating from the heated cover layer. As a result thethickness of the mat increases to approximately 25 mm in the illustratedexample.

[0018] After the heating of the mat to approximately 100° C., thedistance of the pressing planes is adjusted to the final thickness ofthe plate with the effect that the pressure is increased to for instance5 to 10 kp/cm² so as finally to decrease towards 0 at the termination ofthe third phase, viz. the calibration phase.

[0019] The described method is a method known especially within the MDFindustry and it is suited for achieving specific density profiles, cf.FIG. 5. It is, however, encumbered with a few essential draw-backs whichcan be avoided by the use of the invention:

[0020] The high pressure in the first phase presents very highmechanical requirements to the press, and it involves a risk of band androllers being damaged when the mat contains foreign bodies, such ascompact fibre lumps, glue lumps and the like being undetectable by meansof a metal detector.

[0021] The very low pressure in the second phase is necessary due to thepenetration of steam into the middle layer and the heating of saidmiddle layer, but it implies that the glue full-hardens partiallywithout the particles having sufficient mutual contact.

[0022] The terminating compression during the calibration in phase 3 iseven worse for the process because the glue bridges established underthe low pressure in phase 2 are broken under the higher pressure inphase 3.

[0023] All things considered, this method is solely intended forachieving a specific density profile, but it is not suited for achievingan optimum utilization of the binder. Thus the transverse tensilestrength of the plate can vary a great deal, and the damage in themiddle layer is not always associated with the lowest density, cf. FIG.7.

[0024] Various suggestions have been made:

[0025] A drying of the wood material to a low moisture percentage, suchas 5 to 6% followed by a spraying of water on the mat immediately beforethe press. The latter is in principle an efficient method because thepotential amount of steam for the heat transfer is increased withoutincreasing the total amount of moisture and consequently the risk ofsteam burstings. It is, however, difficult to control the procedure, andin addition it is not possible immediately before the press to applywater onto the bottom side of the mat. The result can be asymmetricalcross sections of the plates and curved plates.

[0026] A preheating of the mat by means of high-frequency waves to 50 to60° C. or more in such a manner that the necessity for a heating in thepress is reduced to a level which can be established by means of amoderate “Dampfstoss-Effekt”. The process is difficult to controlbecause even insignificant moisture variations in the mat result in aheterogeneous heating, the dielectric constant of water beingapproximately 80 times higher than the one for wood. In addition, aheating of the middle layer involves a plastifying which is not desiredbecause the middle layer must be able to offer resistance at thecompressing and hardening of the surface of the mat during the firstphase of the pressing.

[0027] A preheating and a setting of the optimum moisture content in themat have furthermore been tested by means of

[0028] superheated steam of a temperature of 110 to 140° C,

[0029] conditioned hot air carried through the mat before the hot pressand of a dew point temperature corresponding to the desired moisturecontent.

[0030] The patent literature discloses several methods based on theabove principles. These methods are characterised by trying to obtain aflow through the mat and consequently a uniform temperature and ahomogeneous moisture content in the entire cross section of the mat.

[0031] The above methods are not advantageous because of the undesiredplastifying of the middle layer and the not-optimum “Dampfstoss-Effekt”,where the moisture content and the temperature are also increased in themiddle layer of the mat, and accordingly it is the object of theinvention to obtain a specific and controllable gradient of the moisturecontent and the temperature in the mat immediately before thecontinuously operating press.

BRIEF DESCRIPTION OF THE INVENTION

[0032] This object is according to the invention obtained by subjectingthe mat immediately before the introduction into the hot press to apretreatment with steam, whereby the length being subjected to the steamtreatment depends on the measured density profile in such a manner thata gradient of the moisture content/temperature is obtained across thethickness of the mat which is optimal with respect to the plastifyingdegree for a desired product quality and a predetermined pressingprocess. As a result, the capacity of the apparatus can be increased atthe same time as the energy consumption is reduced. Furthermore, thedimensions of the press can be reduced.

[0033] Moreover, the mat may according to the invention have atemperature of preferably below 40° C. before the pretreatment.

[0034] Furthermore, the mat may according to the invention have amoisture content of preferably less than 5% relative to the dry weightof said mat before the pretreatment.

[0035] The pretreatment can advantageously be carried out with saturatedwater steam at a temperature of 100 to 115° C., preferably 102 to 110°C., especially in the range 104 to 108° C.

[0036] Moreover, the pretreatment may according to the invention becarried out at a steam pressure of 0.1 to 0.5 bar overpressure,preferably 0.2 to 0.4 bar overpressure.

[0037] The introduction of steam may advantageously be controlled suchthat the gradient of temperature and the moisture content are adjustedto the subsequent hot pressing parameters and the plastifying andcompressing of the mat in order to achieve a predetermined densityprofile of the finished plate. The pretreatment is controlled such thatsteam burstings in the finished plate in the press outlet are avoidedpartly by way of an optimizing of the moisture profile in the mat andpartly by way of keeping the total moisture content in the mat at lessthan 10%, preferably less than 8% of dry weight of the mat.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The invention is explained in greater detail below with referenceto the accompanying drawing, in which

[0039]FIG. 1 illustrates an apparatus in form of a production line forcontinuously producing biomass-based plates, including chipboards andfibre boards,

[0040]FIG. 2 is a side view on a larger scale of the inlet portion ofthe continuously operating press shown in FIG. 1 including an apparatusfor steam processing according to the invention,

[0041]FIG. 3 is a top view of the inlet portion of FIG. 2,

[0042]FIG. 4 shows an example of the temperature course at fourdifferent depths of the mat versus the time and by means of conventionalheating technique,

[0043]FIG. 5A illustrates an example of a density profile of an MDFplate,

[0044]FIG. 5B illustrates a simplified model profile with the same maindata as in FIG. 5A,

[0045]FIG. 6A illustrates an example of pressure and distance control ina continuous hot press according to the prior art,

[0046]FIG. 6B illustrates an example of pressure and distance control ina continuous hot press according to the invention, and

[0047]FIG. 7 illustrates examples of lacking coincidence of density andtransverse tensile strength caused by an inappropriate control of thepress.

BEST MODE FOR CARRYING OUT THE INVENTION

[0048] The invention relates to a method and an apparatus forcontinuously producing plates, such as chipboards, fibre boards and thelike boards, where the raw material in form of biomass particles, suchas wood particles, wood fibres and the like fibres, and applied athermosetting binder is spread on a preforming band into an endless mat,said mat subsequently being pre-compressed in a continuously operatingprepress and then pressed in a continuously operating hot press, whereinthe mat is compressed into the desired thickness of the finished plateand the thermosetting binder is hardened.

[0049] According to the invention the mat is pretreated immediatelybefore the introduction into the hot press E with water steam in such amanner that a specific gradient of the moisture content and thetemperature is obtained which is optimal for a predetermined pressingprocessing and a desired product quality.

[0050]FIG. 1 shows a production apparatus in form of a production linefor continuously producing biomass-based plates, including especially,but not exclusively wood-based chipboards and fibre boards.

[0051] The apparatus F for steam injection is shown in greater detail inFIGS. 2 and 3.

[0052] Above all, the hot press E is of vital importance for thecapacity of a production line and the properties of the products, saidhot press having two basic functions:

[0053] compressing a mat B comprising glued biomass particles into thedesired thickness of the plate,

[0054] heating the mat B to a temperature causing the binder to harden,i.e. polymerising/condensing,

[0055] For this purpose two types of hot presses are used, viz.

[0056] conventional step presses pressing a section of the mat perpressing cycle,

[0057] continuously operating through-type presses, wherein an endlessmat B is carried by means of steel bands 11 through a wedge-shaped slotbetween two pressing planes 12, whereby the mat B is graduallycompressed and full-hardened by the heat from said pressing planes 12and said steel band 11. Such presses have become more and more importantand are expected to dominate the market within a few years. Theinvention is in particular directed towards a use in connection withsuch a press.

[0058] The positioning of the hot press E in the production line isshown in FIG. 1.

[0059] Below reference is only made to a continuous press, and thecapacity of said press depends on

[0060] the capacity of the press to transfer heat from the pressingplanes 12 to the steel bands 11. In this connection, especially theshape of the roller or slide systems 13 between the pressing planes 12and the steel bands 11 is of decisive importance.

[0061] the transfer of heat from the steel bands 11 to the mat of woodparticles and through said mat which is to be heated to approximately105 to 110° C. in the middle in order to harden the binder. In practice,the heat transfer in the mat B turns out to be the limiting factor, Thethermal conductivity in the mat B is very poor, and accordingly attemptshave been made at optimizing the so-called “Dampfstoss-Effekt”, which isa German technical term meaning that the moisture in the surface of themat B evaporates and moves towards the centre of the mat B where thesteam condenses and releases its evaporation heat.

[0062]FIG. 4 shows an example of the temperature course at fourdifferent depths of the mat B versus the time and consequently theposition of the measuring location above the pressing length. The curvesegments with a steep temperature gradient represent the“Dampfstoss-Effekt” in the layer in question, whereas the flattemperature gradients represent the heat conducting phase taking overwhen steam is no longer supplied from the outside.

[0063] It appears that the heat conducting phase requires most time andrestricts the advancing speed and consequently the capacity of the pressE.

[0064] Thus the “Dampfstoss-Effekt” is an ideal mechanism fortransferring heat, but it is of limited use because a high steampressure in the middle layer ML may cause steam burstings in said layerwhen the plate exits the press E. The more steam that is supplied inconnection with the through heating of the mat B, the longer period theplate must remain under a slight pressure in the press E so that thesteam can finally condense or escape from the middle layer ML.

[0065] Thus an optimizing of the capacity of the press E by means ofwater/steam dosing presents a compromise between two counter-actingeffects.

[0066] Various suggestions have been made:

[0067] Drying of the wood material to obtain a low moisture percentage,such as 5 to 6% followed by a spraying of water on the mat immediatelybefore the press E. In principle, the latter is an efficient methodbecause the potential amount of steam for the heat transfer is increasedwithout increasing the total amount of moisture and consequently therisk of steam burstings in the plate. It is, however, difficult tocontrol the procedure, and in addition it is not possible immediatelybefore the press E to apply water onto the bottom side of the mat. Thelatter may result in asymmetrical cross sections of the plate and curvedplates.

[0068] A preheating of the mat B by means of high-frequency waves to 50to 60° C. or more in such a manner that the necessity for a heating inthe press E is reduced to a level which can be established by means of amoderate “Dampfstoss-Effekt”. The process is difficult to controlbecause even insignificant moisture variations in the mat B result in aheterogeneous heating, the dielectric constant of water beingapproximately 80 times higher than the one for wood. In addition, aheating of the middle layer ML of the mat B involves a plastifying whichis not desired because the middle layer ML must be able to offerresistance at the compressing and hardening of the surface DL of the matB during the first phase of the pressing.

[0069] A preheating and a setting of the optimum moisture content in themat B have furthermore been tested by means of various combinations of

[0070] superheated steam of a temperature of 110 to 140° C.,

[0071] conditioned hot air carried through the mat B before the hotpress E and of a dew point temperature corresponding to the desiredmoisture content.

[0072] These methods are characterised by trying to obtain a flowthrough the mat B and consequently a uniform temperature and ahomogeneous moisture content in the entire cross section of the mat B.

[0073] In view of

[0074] The undesired plastifying in the middle layer ML,

[0075] the not-optimum “Dampfstoss-Effekt”, where the moisture contentand the temperature are also increased in the middle layer ML of the matB, the above is not advantageous, and accordingly it is the object ofthe invention to obtain a specific and controllable gradient of themoisture content and the temperature in the mat B immediately before thecontinuously operating press E.

[0076] The method is carried out as follows:

[0077] Immediately before the press E, the pre-compressed mat B issupplied with saturated steam at a temperature of preferably, but notexclusively 105 to 110°0 C. corresponding to an overpressure of 0.2 to0.4 bar. The position of the press E in the production line appears fromFIG. 1. The detailed structure of the press inlet and the apparatus Ffor injection of steam according to the invention appears from FIGS. 2and 3.

[0078] A device comprising a plane below and a plane above the mat B isaccommodated directly in the inlet of the continuously operating pressE, preferably, but not exclusively as an integrated portion of aretractable feeding device D. These planes are provided with channels 2for distribution of steam across the width of the production line, andthey comprise bores in the surface for the feeding of steam to the mat Bbeing advanced between said planes by means of strainer bands 15, i.e.permeable bands made of textile or metal tissue or the like tissue.

[0079] The planes are structured as shown in FIGS. 2 and 3.

[0080] The bottom plane 1 is shaped as a coherent plane with cylindricalchannels parallel to the plane 1, but perpendicular to the introductiondirection of the mat B. The steam is supplied through resilient coils 3to the channels 2 through pistons 4 in form of tubes, cf. FIG. 3. Thetubes can be moved and positioned in the outermost portion of thechannels 2. Steam to the mat B leaks through bores 5 in the surface ofthe planes, and the leaking can be limited by means of the pistons 4 tothe portion of the width of the production line which is relevant for apredetermined production width. The production width can vary accordingto desire by means of the spreading machine A.

[0081] The upper plane is structured correspondingly concerning theintroduction of steam, but it comprises segments interconnected throughhinges 6 with the result that each segment can be pressed downwards bymeans of hydraulic cylinders 7 towards the strainer bands 15 and the matB in such a manner that a leaking of steam between the plane, thestrainer bands 15 and the mat B can be limited.

[0082] The structuring of the planes for steam processing in form ofmodules allows a simple adjustment of the capacity, viz. the lengthbeing processed, to the instant advancing speed associated with thelength and capacity of the hot press E in question.

[0083] The supply of steam can be adjusted to each segment or to eachchannel 2. The pressure and the temperature can also be adjustedindividually.

[0084] In this manner the penetration of steam and the heating can becompletely or partially limited to the cover layer in accordance with aprofile which can be maximally adjusted to a predetermined processingand a desired product quality.

[0085] An in-line determination of the density profile in the finishedplate after the hot press E is used as auxiliary means for theadjustment of the moisture and the temperature profile in the mat B, cf.FIG. 1.

[0086] Correspondingly the detector H is used as auxiliary means for thecontrol of the total supply of moisture to the mat B, said detectorappearing from FIG. 1 and detecting a possible formation of blisterscaused by a too high steam pressure.

[0087] The effect of the above setting of a specific moisture andtemperature profile in the cross section of the mat B is illustrated bymeans of a calculation example performed on a typical quality of MDF(Medium Density Fibreboard) with an average density of 800 kg/m³ and adensity profile as shown in FIG. 5A.

[0088] In order to simplify the calculations, this profile has beenreplaced by a geometrically formalized profile with the same main dataas the actual profile, cf. FIG. 5B.

[0089] The layer structure of the plate is as follows:

[0090] The cover layer DL dividable into three layers:

[0091] a loose layer DL1 resulting from a prehardening of the surfacebefore a full pressure has been established, here assumed with athickness of 0.5 mm and an average density of 550 kg/m³. This layer isusually buffed off.

[0092] DL2, density maximum, the thickness is here 0.5 mm, the averagedensity is 1100 kg/m³.

[0093] DL3, transition to the middle layer ML, here assumed with athickness of 3 mm, density 1100→700 kg/m³.

[0094] The middle layer ML, thickness 9 mm, average density 700 kg/m³,

[0095] Cover layer DL identical with the above layer.

[0096] After the buffing off of the loose surface, the thickness is 16mm, and the total density is 800 kg/m³.

[0097] For this purpose a fibre mat is required, said mat in thefollowing calculations being divided into a cover layer DL, and a middlelayer ML corresponding to the finished plate.

[0098] The mat is assumed to be spread with a moisture content of 5% anda temperature of 40° C., said temperature having dropped in the surfaceto 30° C. on the way from the spreading station A to the press E. Themat before The mat after The plate after steam processing steamprocessing hot pressing P = 0.2 to 0.4 bar t = 105 to 110° C. Q = 540kcal/kg STEAM

P = 0.2 to 0.4 bar t = 105 to 110° C. Q = 540 kcal/kg STEAM

COVER LAYER DL surface weight t = 30° C. t = 95° C. t = 130 to 140° C.3.525 kg/m² ^(c)fibres = 0.45 kcaf/kg u = 5% u = 5% + Δu u = 5% MIDDLELAYER ML surface weight t = 40° C. t = 40° C. t = 112° C. 6.300 kg/m²^(c)fibres = 0.45 kcal/kg u = 5% u = 5% u = 11.1% COVER LAYER DL surfaceweight t = 30° C. t = 95° C. t = 130 to 140° C. 3.525 kg/m² ^(c)fibres0.45 kcal/kg u = 5% u = 5% + Δu u = 5%

STEAM P = 0.2 to 0.4 bar t = 105 to 110° C. Q = 540 kcal/kg

[0099] It is assumed that the cover layers are heated by means ofsaturated water steam to 95° C. The latter requires$\frac{2 \cdot 3.525 \cdot 0.45 \cdot 65}{540} = {0.382\quad {{kg}/{steam}}}$

[0100] whereby the moisture content in the cover layer in DL isincreased to${{5\%} + {\frac{0.382 \cdot 100}{2 \cdot 3.525}\%}} = {10.4\%}$

[0101] A transfer of the supplied amount of steam to the middle layer MLby way of heating in the press E results in a moistening in the middlelayer ML to${\left( {5 + \frac{0.382 \cdot 100}{6.3}} \right)\%} = {11.1\%}$

[0102] and a heating to${\left( {40 + \frac{0.382 \cdot 540}{0.45 \cdot 6.300}} \right){^\circ}\quad {C.}} = {112{^\circ}\quad {C.}}$

[0103] Thus the heat supply solely by way of the “Dampfstoss-Effekt” iscompletely sufficient for hardening the glue in the middle layer ML. Inaddition, a resulting moisture percentage of 11.1% in the middle layerML and a total moisture percentage in the mat B of${\left( {5 + \frac{0.382 \cdot 100}{13.350}} \right)\%} = {7.86\%}$

[0104] are completely non-critical with respect to the risk of steamburstings in the finished plate at the press exit. A particular coolingzone in the press E is therefore not necessary.

[0105] The hot pressing in a continuous press by way of the methodaccording to the invention runs typically in the following manner, cf.FIG. 6B-1 illustrating the pressure course across the length of thepress, and FIG. 6B-2 illustrating the distance of the pressing planesacross the pressing length.

[0106] a preheating of the outer layer of the mat to for instance 103°C. by a steam pressure of 0.1 bar overpressure and a moistening to forinstance 10 to 12% result in an intensive plastifying of fibres/chips,and at contact with the hot pressing bands (t>200° C.) this effect isadditionally enhanced.

[0107] Thus the pressure necessary for achieving a high surface density(1000 to 1100 kg/m³) can be reduced by a factor of the magnitude 3 to 4or more.

[0108] The low pressure in the first phase has the effect that themiddle layer of the mat is less compressed than by the conventionalmethod. Accordingly, during the entire pressing procedure the middlelayer is d permeable to the penetrating steam from the cover layer, andtherefore the heating of said middle layer is carried out very quicklyand simultaneously under a pressure providing better possibilities for acontact between the particles during the hardening of the glue than bythe conventional technique.

[0109] The pressing procedure runs typically as follows:

[0110] During phase 1 a pressure is established, which typically is ofthe magnitude 10 to 15 kp/cm², which according to the density profilemeasurements ensures the desired density maximum, typically 1000 to 1100kg/m³.

[0111] This pressure is maintained until the cover layer has achievedthe desired thickness. The time necessary is also determined by way ofdensity profile measuring.

[0112] The pressure is reduced in phase 2 according to a homogenouslydecreasing curve, the outline of which is decisive for the structure ofthe density profile in the middle layer of the plate. The thickness ofthe mat is registered as a secondary parameter.

[0113] When the mat has reached the final thickness of the plate, thedistance of the pressing planes take over as primary control parameterin phase 3.

[0114] The distance is maintained on the final thickness of the plate,and the pressure is registered as a secondary parameter. When thepressure approaches 0, the plate is hardened and the pressingterminated.

[0115] The advancing speed of the mat can be adjusted to the specificpressure in the press in phase 3. When the pressure drops to 0 directlybefore the exit, the speed is suitable. When the pressure drops earlierto 0, the speed can be accelerated without risking steam burstings.

[0116] The entire method involving both an establishment of a specificmoisture and temperature profile in the thickness of the mat and theillustrated pressing and temperature profile in the continuous hot pressensures the following advantages over the conventional technique:

[0117] The heat transfer from the surface of the mat B to the middlelayer ML is almost exclusively performed by means of steam from thecover layer DL. As the temperature in the cover layer DL is alreadyclose to the boiling point of the water, the “Dampfstoss-Effekt” isinitiated very quickly by a contact with the up to 200° C. hot pressingbands.

[0118] Almost half the heat energy necessary for hardening the binder isthus supplied in a simple manner before the mat B is introduced into thepress E, which represents the most expensive component andsimultaneously the capacity-limiting member of the production apparatus.

[0119] The low maximum pressure in the press inlet ensures a reducedenergy consumption by the compression of the mat and a reduced wear ofthe mechanical parts of the press.

[0120] The use of a pressing procedure involving a moderate,homogenously decreasing pressure ensures the best possible conditionsfor utilizing the binder and for achieving the best possible transversetensile strength in the middle layer of the plate.

[0121] The use of a moderate compression of the mat during the firstphase of the pressing procedure ensures the best possible permeabilityfor the steam from the cover layer and consequently the fastest possibleheat transfer to the middle layer.

[0122] The total capacity of the apparatus can thus be substantiallyincreased while the energy consumption is simultaneously reduced. As analternative the size, dimensioning and hydraulics of the press E can bereduced to a predetermined capacity.

[0123] The latter is also ensured because the pressure in the hot pressE can be substantially reduced by the cover layer being plastifiedbefore the pressing.

[0124] By setting a specific profile of the temperature and the moisturecontent in the mat B before the pressing procedure, it is possible toefficiently control the plastifying and compressing procedure in the matB during said pressing. In other words it is possible to obtain anadditional possibility of controlling the density profile and otherproperties of the end product beyond the possibilities provided by thehot press E per se.

[0125] By adjusting and controlling the moisture content and themoisture profile before the hot pressing it is possible to ensure a morereliable basis for the function of the continuous hot press E than bythe existing control systems.

[0126] The use of an in-line density profile measuring at G and adetection of blisters at H on the finished plate after the press Erenders it possible to obtain a direct and clear connection between theprocess parameters and the product properties, and accordingly it ispossible to obtain the desired product properties.

[0127] The length being subjected to the steam processing is typically 1to 2 m, but it depends on the advancing speed and the thickness of theplate.

1. A method of optimizing the production capacity and the flexibility ofthe product properties when manufacturing chipboards, fibre boards andthe like boards by a continuous process, where a thermosetting binder isapplied onto the raw material in form of biomass particles, such aschips, fibres and the like, said raw material being spread on apreforming band into an endless mat (B), said mat (B) is preferablypre-compressed in a continuously operating prepress (C) and completelypressed in a preferably continuously operating hot press (E), and wheresaid mat (B) is compressed into the desired thickness of the finishedplate and the thermosetting binder is hardened, characterised in thatimmediately before the introduction into the hot press (E) the mat (B)is pretreated with steam, whereby the length being subjected to thesteam processing depends on the measured density profile in such amanner that a gradient of the moisture content/temperature is obtainedacross the thickness of the mat (B) which is optimal with respect to theplastifying degree for a desired product quality and a predeterminedpressing process.
 2. A method as claimed in claim 1, characterised bythe mat (B) having a temperature of preferably below 40° C. before thepretreatment.
 3. A method as claimed in claim 1 or 2, characterised bythe mat (B) having a moisture content of preferably less than 5%relative to dry weight of the mat before the pretreatment.
 4. A methodas claimed in claims 1 to 3, characterised by the pretreatmentpreferably being performed with saturated water steam at a temperaturein the range 102 to 110° C., preferably in the range 102 to 110° C., andespecially in the range 104 to 108° C.
 5. A method as claimed in claims1 to 4, characterised by the pretreatment being performed at a steampressure of 0.1 to 0.5 bar overpressure, preferably at a steam pressureof 0.2 to 0.4 bar overpressure.
 6. A method as claimed in claims 1 to 5,characterised by the introduction of steam being controlled in such amanner that the gradient of steam and moisture content is adjusted tothe following hot pressing parameters (temperature, pressure andpositioning of the pressing planes across the pressing length) and thedesired plastifying and compressing procedure of the mat (B) in order toobtain a predetermined density profile of the finished plate, thepretreatment being controlled such that steam burstings in the finishedplate in the press outlet are avoided partly by way of an optimizing ofthe moisture profile in the mat (B) and partly by way of keeping thetotal moisture content in the mat (B) at less than 10%, preferably lessthan 8% of dry weight of the mat.
 7. A method as claimed in claims 1 to6, characterised by using the density profile in the finished platemeasured (at G) immediately after the exit of the hot press (E) ascontrol parameter for the setting of pressure, temperature and period ofthe pretreatment by means of steam.
 8. A method as claimed in claims 1to 6, characterised by controlling the dosing of the total amount ofsteam used at the pretreatment on the basis of an ultrasonic measuring(at H) of beginning steam burstings in the finished plate at the exit ofthe hot press (E).
 9. An apparatus for carrying out the method asclaimed in claim 1, characterised in that it comprises a steam injectiondevice (F) divided into preferably interconnected segments, and that thelength/period of processing in the steam injection device (F) can beadapted to the mat (B) in question and the desired moisture andtemperature gradients by way of a connection/disconnection of theindividual segments, and optionally a connection/disconnection of theindividual channels in each segment.
 10. An apparatus as claimed inclaim 9, characterised in that the individual segments can be presseddownwards by means of hydraulic cylinders (7) towards the mat (B). 11.An apparatus as claimed in claim 9, characterised in that the steaminjection device (F) comprises a few movable pistons structured ascylinders (4) for the introduction of steam into cylindrical channels(2), said pistons allowing an adjustment of the width of the processingzone in accordance with the width of the mat (B).
 12. A method asclaimed in claims 1 to 11, characterised by the following procedure ofthe hot pressing in a first phase a pressure is established, which istypically of the magnitude 10 to 15 kp/cm², said pressure beingmaintained until the cover layer has reached the desired thickness, in asecond phase the pressure is reduced according to a homogenouslydecreasing curve, the outline of which is decisive for the structure ofthe density profile in the middle layer of the plate, and then when themat has reached the final thickness of the plate, the distance betweenthe pressing planes takes over as primary control parameter until thepressure approaches 0.